Phase change random access memory (PCRAM) refers to a non-volatile memory device capable of recording and reading data according to an applied current. In a PCRAM device a volume of phase change material is deposited between two electrodes to form a single memory cell. Phase change materials are used in electronic memory applications because of their ability to electrically switch between an amorphous and crystalline state. These materials selectively exhibit more than one value of electrical resistivity. For example, when the phase change material is in a crystalline state its resistance is low, and when it is in an amorphous state its resistance is high.
In a PCRAM device, a programming current is passed through the phase change material to induce a phase change. This programming current generates heat as a result of the electrical resistance of the phase change material. The amount of heat generated is proportional to the current density in a fixed volume of material. As the volume of material is decreased the programming current required to induce the phase change also decreases. Furthermore, as the programming current is reduced the amount of heat generated also decreases.
Since each memory cell utilizes a programming current, and there are millions of memory cells per PCRAM device, a large overall energy input is required to operate the device. It is desirable to reduce the amount of programming current required to induce the phase change and, in so doing, reduce the total energy requirements of the device.
Additionally, there is an increasing need to produce ever-smaller memory devices. As memory devices are condensed, the relative distance between neighboring memory cells is lessened, resulting in cells of extremely close proximity. It is theorized that cells in such close proximity will be subject to increased thermal influence from adjacent cells. This phenomenon is known as “thermal cross-talk.” Thermal cross-talk occurs when heat generated in one memory cell, by application of the programming current, is thermally conducted to an adjacent memory cell.
Thermal cross-talk is undesirable because it can cause an unwanted phase change in a memory cell, resulting in corruption of the data stored within the memory cell. Transitions between the amorphous and crystalline states may be initiated by temperature change. If thermal cross-talk is not prevented, it is possible that the phase change material of an unselected cell, one to which current is not applied, will be transformed (i.e., inadvertently programmed to an incorrect state) due to heat transfer from an adjacent cell. It would be desirable to form a device capable of operating with reduced energy draw and negligible thermal cross-talk despite minimal scale and high cell density.
U.S. Patent Application Publication No. 2007/0181932 to Happ et al. describes a method of thermally isolating phase change memory cells. Adjacent phase change memory cells are separated from one another by first and second insulating materials. The phase change materials in the phase change memory cells have an hourglass or tapered shape.